Desulfurization of coal

ABSTRACT

A method for carbonizing and desulfurizing carbonaceous materials by heating the carbonaceous material admixed with iron in a reducing atmosphere between 1,200* and 1,700*F and, subsequently, subjecting the resulting char to an oxidizing atmosphere at a temperature between 800*-1100*F. The resulting product is a carbonized, desulfurized fuel which has a sulfur content of 0.6 to 0.7% by weight.

United States Patent Selmeczi et al.

14 1 May 27, 1975 DESULFURIZATION OF COAL 3369,87] 2/[968 Hardy et al. 1. 20l/l7 X 3,597,327 8/1971 Squires 3,736.233 5/1973 Sass et al. 20l/3l Primary Examiner Norman Yudkoff [73] Assignee: Dravo Corporation, Pittsburgh Pa. Assistant ExaminerD. Sanders 22] Fngdz g 17, [973 grrrginey, Agent, or F1rmParmelee, M1ller, Welsh & [2]] Appl. No: 389,338

[57] ABSTRACT [52] Cl 0 201/20 A method for carbonizing and desulfurizing carbona- [5 H 3" g 57/00 C [mas-H04 ceous materials by heating the carbonaceous material [58] held of Search admixed with iron in a reducing atmosphere between 9 l,200 and 1,700F and, subsequently. subjecting the resulting char to an oxidizing atmosphere at a temper- [561 Reierences cued ature between 800l lOOF. The resulting product is a UNITED STATES PATENTS carbonized. dcsulfurized fuel which has a sulfur con- 2.72l ,l69 10/1955 Mason et al. 2Ul/l7 X tent of 0.6 to 0.7% by weight.

2,824.047 2/1958 Gorin et al 1 1 .1 201/20 2,919,231 12 1959 Donath 201/20 x 6 Clams. 4 Drawmg Flgures GAS 600 BTU/CF TAR 0/1.

FLUE r0 CHAR l $0 GAS STRIPP/NG CHAR 1 ,8 HOT CHAR OX/DIZED c041. H1 g COLD CHAR GOAL ,2 Fe-FeO AIR PRODUCT PATENTED MAY 2 7 I975 SHEET mPfiSm EGG m m mm A QQLQ ay /I 51 8 QSQ IT MM I. Q36 8: A A E ||A| $5 om is m5 kUbQQQl xm k 1 DESULFURIZATION or COAL CROSS-REFERENCE TO RELATED APPLICATION An application entitled Carbonization and Desulfurization of Coal by the inventors herein, and also relating to desulfurization of coal, was filed simultaneously with this application, and the contents thereof are incorporated by reference herein.

BACKGROUND OF THE INVENTION It Field of the Invention Carbonized coal products that are low in sulfur content are produced from sulfur containing coals. A major part of the sulfur is removed from the coal prior to normal combustion of the coal to produce energy, thus reducing the amount of sulfur compounds released on combustion thereof.

2. Prior Art Because of the natural occurrence of sulfur in raw coal, large quantities of sulfur compounds are discharged into the atmosphere whenever coal or coke is burned as a source of heat energy. These sulfur compounds are some of the most objectionable air pollutants. As such, their emission into the air has been regulated by the Federal government, as reported in Slandards of Performance for New Stationary Sources, Federal Register, Aug, 197 l. These standards require that a coal with a [3,000 Btu/lb. thermalvalue may contain not more than 0.8% sulfur for use without an air pollu tion control equipment installation. Although processes have been developed to remove sulfur compounds from stack gases following combustion of coal, the removal of sulfur from coal prior to combustion has not been economically achieved, probably because of the nature of the sulfur present in coal, which is in both organic and inorganic form.

The reserves of low-sulfur content coal, i.e., coal containing less than about I% sulfur, are being steadily diminished in the industrial Eastern section of the country, as discussed in the publication, Sulfur Content oflhe United States Coals, J. A. DeCarlo, E. T. Sheridan, and E. E. Murphy, information circular 8312 by the Bureau of Mines, US. Department of the Interior, where at page 4 it is stated that almost 80% of the reserves in mines east of the Mississippi contain more than 3% sulfur. A large portion of this coal contains as much as 5% sulfur. Because of these factors, there is a need for coal or coke products lower in sulfur content than those which can be obtained from natural sources. Previous methods for lowering the sulfur content of 5% sulfur coal have not been able to achieve a sulfur content of less than about I%.

It is an object of this invention to attain the conversion of coal into a low sulfur char for use in power plants, and high Btu gas suitable for use as a fuel.

It is a further object of this invention to produce a char for use as a power plant fuel which has a sulfur content low enough to permit its use in power plants, without air pollution control equipment, and still meet Federal sulfur emission standards.

SUMMARY OF THE INVENTION The present invention is a method of carbonizing and desulfurizing carbonaceous materials. The carbona ceous material is first admixed with iron or iron oxide such that the percentage of iron is between 3 and 30% of the mixture by weight. It is then heated in a reducing atmosphere between l,200 and 1,700F for between 5 and 60 minutes. The iron is then separated from the resulting char, the char is subjected to an oxidizing atmosphere at a temperature between 800 and l, l 00F, and the char is finally cooled. The separation of the iron from the char may be carried out by either gravitational or magnetic separation. The resulting product is a char with a sulfur content of less than 1% by weight, and preferably about 0.6% by weight, which is directly usuable as fuel.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic representation of the method of the present invention, and of apparatus used with this method.

FIG. 2 is a graph showing the influence of the coal preheating temperature on char reactivity during the selective sulfur oxidation at 900F with Air-N mixture (11l) for 30 minutes.

FIG. 3 is a graph of the influence of the oxidation temperature on the selective sulfur removal and carbon burn-off of the coal which was charred at l.600F. l00 mesh char was fluidized with Air-N mixture (lzl) for 30 minutes.

FIG. 4 is a graph of the influence of reaction on the sulfur removal and carbon burn-off during the oxidation of coal which was charred at l,600F. -l00 mesh char was fluidized with Air-N mixture (lzl at 900F.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing, in one embodiment, finely divided coal which is 65% total carbon, 40% volatile material and 5% total sulfur is introduced into a fluid-state reactor 10, such as a conventional fluidized bed, along with air heated to about 600F by a gas preheater 12 for decaking. After decaking the coal is admixed with finely divided iron in an amount of 30% by weight, in the form of iron or iron oxide or their mix which may originate from coal pyrites, and introduced into a second fluid-state reactor 14 for about 30 minutes. The coal is carbonized and partially desulfurized in reactor 14. The FeS is converted to more stable FeS and also to H 8 which may be either in free form or partially adsorbed on the carbon surface. These reactions are explained in more detail below.

The sulfided iron-oxide is gravitationally separated from the resulting char in the second fluid-state reactor 14 or alternatively the iron-oxide may be magnetically separated from the char in a separate step. The resulting char is then introduced into a two-stage third fluidstate reactor 18 where it is oxidized at about l,000F in the first stage and discharged hot to be burned in a boiler or cooled in the second stage by the introduction of air. The resulting char has a carbon content of volatile material of 1% and a total sulfur content of 0.6% by weight. It also has a thermovalue of l l,000l 2,000 Btu/lb. After separation from the char, the sulfided iron may be introduced into an iron regenerator 20 where it is regenerated with steam saturated hydrogen-containing gas and recycled to be admixed with further coal or removed as by-product iron, The off gases from the second fluid-state reactor 14 are passed through a tar-oil separator 22 to produce byproduct taroil and further off gases. The hydrogencontaining gas from iron regenerator 20 is separated from hydrogen sulfide and sulfur in separator 24. The

hydrogen sulfide is mixed in a sulfur production furnace, such as conventional Claus furnace 26, with the sulfur dioxide from the third fluid-state reactor to produce by-product sulfur. After separation in the tar-oil separator 22, the usable fuel gases comprise approximately 50% hydrogen, 35% methane and 15% other gases, which could be further enriched in a methanator to produce synthetic natural gas (SNG).

The initial introduction of 2,000 lbs. of coal in this embodiment results in l,OO-l,200 lbs. of fuel char, 100 lbs. of tar-oil, 80 lbs. of sulfur, 14000 SCF of fuel gas and [00 lbs. of by-product Fe. The total percentage ofiron used may be between 3 and 30% by weight, and the reducing time may be between 5 and 60 minutes. These ranges are determined by the initial sulfur content of the coal and are considerably more limited for coal of any particular sulfur content. The temperature range in the reducing stage must be greater than l,200F to achieve the desired reaction but less than l,700F to prevent agglomeration of the iron.

For proper operation of the fluidized bed, the iron and coal particles must be of sufficient fineness to pass through l00 mesh screen. This fineness is also necessary to insure intimate contact of the iron and coal particles, and consequently greater reactivity.

The oxidation with air of various types of chars and a 5% sulfur steam coal in a temperature range 600l ,200F were tested with different degrees of success of sulfur removal. The best desulfurization results were achieved on chars which were prepared at ],600F in various gaseous environments and in the presence of sulfur acceptors. The optimum temperature range for selective oxidation of sulfur from chars was found to be 800l ,000F.

The important discovery is the effect of metallic iron on the sulfur behavior during carbonization. An addition of 30% by weight iron powder to the coal blockaded the transformation of the main part of the sulfur into the locked-in type of organo-complexes. Consequently, the mass of the remaining sulfur was easily removed by selective oxidation. The final sulfur content in the oxidized char was reduced to 05-07% sulfur. These results indicate that iron has a greater effect than that of a simple sulfur acceptor such as dolomite.

Tests of coal thermal pretreatment prior to the oxidation step revealed that carbon reactivity and sulfur forms have a decisive influence on the reaction selectivity. In the course of heating the coal to 1,600F, thermo-cracking produced an evolution of carbonization gas and a change of the structure of char, making it less susceptible to air oxidation. Parallel to the changes of the carbonaceous matter during carbonization, the sulfur compounds underwent a deep pyrolysis. The pyrites (FeS changed into stable FeS, and the liberated sul fur, in elemental form or as H 5, was partially adsorbed on the carbon surface, which reactions are shown be low:

2 Fes, 2 FeS+ [5 +5 Similarly, the original organic coal sulfur was transformed into a stable organo-complex, and the more volatile part was released as H 8.

The second generation of organic sulfur compounds [CSC] was formed at high temperature from the reaction of surface adsorbed sulfur [S] and carbon:

The oxidation tests of pretreated coals revealed that FeS and surface sulfur complexes are highly reactive with oxygen, and may be preferentially removed from the coal. The residual sulfur in oxidized chars was found to be of the insoluble locked-in organic type The char oxidation may be described by the following reactions;

1.2 FeS 3.5 0 Fe 0 5. C /2 O CO Reaction;

1. Represents the oxidation of inorganic sulfur.

2. Oxidation of sulfur surface-complexes [S].

3. Oxidation of sulfur organo-complexes 4. & 5. Char carbon oxidation.

The minimization of reactions 3, 4 and 5, which present the carbon losses, is the key to the success of this method.

The function of iron in the process of hydro-desulfurization of coal is complex. Iron performs not only as a sulfur acceptor but also as a sulfur compound modifier. The iron in the process of coal hydro-desulfurization alters the type of the residual char sulfur making it highly susceptible to selective oxidation with air. The process of hydro-desulfurization of coal in the presence of metallic iron followed by selective oxidation of char improves significantly the degree of coal desulfurization. This process converts 5% sulfur coal into a 0.6% sulfur char which can be used as a fuel without air pollution control equipment.

What is claimed is:

l. The method of carbonizing and desulfurizing coal which has been decaked comprising:

a. heating the decaked coal admixed with a desulfurizing material selected from the group consisting of iron, iron oxide and the mixture thereof, produced from coal pyrites, which desulfurizing material has been regenerated by contact with a hydrogencontaining gas, such that the percentage of the desulfurizing material is between 3 and 30% by weight of the admixture, in a reducing atmosphere between l,2001,700F for between 5 to 60 minutes to form a partially desulfurized char;

b. separating the desulfurizing material from the resulting char; and

c. subjecting the resulting char to an oxidizing atmosphere at a temperature of between 800l,l00F, whereby a desulfurized char is produced which contains less than one percent sulfur.

2. The method of claim 1 wherein the temperature of the reducing atmosphere is about l,600F.

3. The method of claim 1 wherein the percentage of said desulfurizing material is between 20 and 30% by weight of the admixture.

4. The method of claim 1 wherein the separated desulfurizing material is regenerated by contact with a reducing gas and at least a portion of the regenerated desulfurizing material is recycled for admixture with further decaked coal.

5. The method of claim 1 wherein said admixture is heated in the reducing atmosphere for between 20-30 minutes.

6. The method of claim 1 wherein said desulfurized char is directly burned as fuel, without cooling thereof,

after being subjected to said oxidizing atmosphere. 

2. The method of claim 1 wherein the temperature of the reducing atmosphere is about 1,600*F.
 3. The method of claim 1 wherein the percentage of said desulfurizing material is between 20 and 30% by weight of the admixture.
 4. The method of claim 1 wherein the separated desulfurizing material is regenerated by contact with a reducing gas and at least a portion of the regenerated desulfurizing material is recycled for admixture with further decaked coal.
 5. The method of claim 1 wherein said admixture is heated in the reducing atmosphere for between 20-30 minutes.
 6. The method of claim 1 wherein said desulfurized char is directly burned as fuel, without cooling thereof, after being subjected to said oxidizing atmosphere. 